In the manufacture of wet process phosphoric acid, no matter how pure the raw materials used in its manufacture may be, the phosphoric acid thereby produced, if contemplated for use in food and high grade chemical purposes, must be purified. The procedure and number of steps required in the purification of wet process phosphoric acid depend upon the method employed in manufacturing the acid and the ultimate use of the products. For example, technical grade phosphoric acid is suitable for most applications except for food and pharmaceutical purposes. A typical analysis appears in the table below:
______________________________________ TYPICAL ANALYSES OF PHOSPHORIC ACID.sup.1 ______________________________________ Crude Acid (Green Acid) Technical Grade Constituent (Wt. %) (Wt. %) ______________________________________ P.sub.2 O.sub.5 32 32 SO.sub.4 2.50 0.10 Si 0.06 0.03 F 0.20 0.05 Cl 0.02 0.002 Fe 0.60 0.003 Al 0.50 0.003 Ca 0.80 0.002 Mg 0.30 0.001 As 0.001 0.001 Pb 0.005 0.001 ______________________________________ .sup.1 Slack, Phosphoric Acid, Vol. I, Part II, P. 722 (Marcel Dekker - 1968).
Impurities commonly found in wet process phosphoric acid include dissolved and suspended materials, both organic and inorganic in nature. Sources of these contaminants come from phosphate rock, reagents used during the beneficiation of the rock, the sulfuric acid, and processing equipment that is physically and chemically attacked during manufacturing steps. The relative amounts of these impurities, therefore, vary considerably among producers who use different raw materials and production methods.
Phosphate rock is the principal source of most of the many dissolved impurities in wet process acid. More than 50 chemical elements have been identified in sedimentary and igneous apatitic ores. However, only a relatively small number are troublesome impurities in conventional wet process acid manufacture. These create sludge. This important group of interfering impurities are almost universally distributed in sedimentary phosphate rocks from nearly all geographical sources, but they do not occur in the same mineral forms in all ores. Therefore, because of the universal distribution of the few important impurity elements, most wet process acid producers have some sludge problems in common, even though their phosphate ores are obtained from widely separated and unrelated sources.
Collectively, the chemical compositions of the sludge solids which have been reported in the literature include iron, aluminum, fluorine, silica, calcium, magnesium, potassium and sodium from phosphate rock, and sulfur trioxide and P.sub.2 O.sub.5 from the acids.
Secondary sources of impurities also occur as chemical additives in beneficiation, in the sulfuric acid, and by the corrosion and erosion of equipment. These differ with each producer. For example, additional organic matter may be introduced into the wet process acid as organic dispersants, flotation agents, and conditioners that are adsorbed on beneficiated concentrates or as anti-foaming agents added in the acidulation step. The sulfuric acid can also contribute to the impurity content by containing small amounts of Fe, Al, Ca, Mg, Si, Mn, Cu, Zn, Pb, and As as have been identified in smelter acids which are frequently used in wet process acid manufacture. Metallic corrosion products from process equipment, particularly iron, may also contribute significantly to the amounts of dissolved or suspended impurities.
No systematic classification of the impurities contributed by these secondary sources is possible, since processing steps and types of chemical reagents differ widely among producers.
Excessive amounts of impurities can have several adverse effects on the phosphoric acid produced. Among these include the impartation of undesired color or turbidity, change of physical characteristics such as viscosity and density and, as already noted, sludge formation.
Numerous methods have been proposed for purifying wet process acid, however, none have been completely effective, although progress has been made. Various methods have been used, including settling and centrifugation, often with the use of flocculating agents. Other methods proposed include the addition of fluosilicic acid to remove potassium, the use of chelating agents to sequester iron and aluminum, high shear agitation, rapid cooling, addition of ammonia, addition of free sulfuric acid, addition of sludge as seed material, and the addition of a gelling type clay to restrict crystal growth and suspend precipitated impurities. None of these operations appear to be in widespread use.